Electrostatic loudspeaker with constant current drive

ABSTRACT

With the objective of obtaining a far field axis pressure which is frequency-independent, an electrostatic loudspeaker that is acoustically transparent over the frequency range of interest, for instance the audio frequency range, comprises separately accessible coplanar sections or individual units arranged to carry a common polarizing charge per unit area and which are connected as the parallel components of an electrical network so organized that the vector sum of the currents in the individual sections or units is always directly proportional to the total current at the input terminals of the network.

United States Patent 1 Walker 1 Nov. 20, 1973 [5 ELECTROSTATIC LOUDSPEAKER wITl-l 2,634,335 4 1953 Stolaroff 179 1002 K CONSTANT CURRENT DRIVE 2,843,671 7/1958 Wilkins 179/1 F 2,905,761 9/1959 Wilins Wilkins/l F [76] Inventor: Peter James Walker, Huntingdon,

England FOREIGN PATENTS OR APPLICATIONS 537,931 71941 G B 'ta' 179111 22 Filed: Nov. 3, 1970 n m I 211 App] 14 Primary Examiner-Kathleen H. Claffy Assistant Examiner-Jon Bradford Leaheey Related Apphcatlon Dam AttorneyFriedman & Goodman [63] Continuation of Ser. No. 736,755, June 13, 1968,

[52] US. Cl 179/111 R [51] Int. Cl H04r 19/02 [58] Field of Search 179/1 F, 111 R, 106 R, 179/100.2 K; 330/28 [56] References Cited UNITED STATES PATENTS 2,302,493 11/1942 Dome 179/1 F 3,135,838 6/1964 Wright 179/111 1,983,377 12/1934 Kellogg 179/111 3,061,675 10/1962 MacDonald. 179/1 F 2,387,845 10/1945 Harry 179/1 F 3,542,952 11/1970 Wang 179/1 F 7900/0 l/VPUT 9 1 1 23%;??? 3 SOUECE F T abandoned.

[57 ABSTRACT With the objective of obtaining a far field axis pressure which is frequency-independent, an electrostatic loudspeaker that is acoustically transparent over the frequency range of interest, for instance the audio frequency range, comprises separately accessible coplanar sections or individual units arranged to carry a common polarizing charge per unit area and which are connected as the parallel components of an electrical network so organized that the vector sum of the currents in the individual sections or units is always directly proportional to the total current at the input terminals of the network.

7 Claims, 2 Drawing Figures PAIENTED uuv 20 I975 Inventor PETER JAMES WALKER By %LIMm E Atlarne y ELECTROSTATIC LOUDSPEAKER WITH CONSTANT CURRENT DRIVE This is a continuation of US. Pat. application Ser. No. 736,755, filed June 13, 1968, and now abandoned.

This invention concerns loudspeakers and is particularly concerned with high-quality electrostatic loudspeakers.

ln high-quality direct radiator loudspeakers the area of the cone or diaphragm must be sufficiently large to radiate adequate power at low frequencies without distortion. However, a cone or diaphragm meeting this area criterion is directional at high frequencies so that it is usual to employ a system of two or three cones or diaphragms or separate loudspeaker units, these being interconnected mechanically or electrically so that sound is radiated from a smaller area as the signal frequency increases.

ln such a system, difficulties are experienced in maintaining a correct balance between the cones, diaphragms or units; in avoiding acoustic interference therebetween; and in ensuring that each cone, diaphragm or unit of the system is adequately prevented from significantly contributing to the radiation of sound at frequencies outside the range of frequencies intended to be radiated by it.

Whilst these problems are common to all types of loudspeakers, this invention is concerned with the mitigation of these problems in an electrostatic loudspeaker.

As is well known, any electrostatic loudspeaker element may take a variety of constructional forms but for the purposes of explaining the present invention it will be convenient to consider the construction that consists essentially of two perforated rigid plate electrodes in parallel relationship with a flexible membrane or diaphragm stretched or supported in the space between the electrodes. If a polarizing charge is fed to the diaphragm and an AC signal is applied between the electrodes there will be a force on the diaphragm proportional to the product of the AC and DC fields and the fold diaphragm will move in response to this force and radiate sound,

An electrostatic loudspeaker element of this construction is acoustically transparent over a wide frequency range between a low frequency limit determined by the stiffness or tension of the diaphragm and a high frequency limit determined by the mass of the diaphragm and any added mass due to the holes in the rigid electrodes. It is comparatively simple to arrange that these limits are beyond, or at the extremes of, the frequency range of interest, e.g. the audio frequency range.

If such an acoustically transparent element were placed in a sound field consisting of a plane wave normal to the surface of the element, it is clear that the element would not disturb the field and the diaphragm would move in accordance with the particle displacement of the wave. If, in such circumstances, the diaphragm carried a charge, a voltage would appear on the electrodes and for the open circuit case this voltage would be proportional to the wave particle displacement and independent of frequency. By application of the reciprocity theorem it can be deduced that if such an electrostatic loudspeaker element is fed from a constant current generator it will produce a far field axis pressure which is independent of frequency.

It is important to note that this behaviour of such an acoustically transparent element is independent of the area of the element and is also independent of its shape. The area and shape of the element will, of course, have a large influence on the directivity of the radiation pattern.

Suppose now that a number of such acoustically transparent electrostatic loudspeaker elements, each of any shape or size, are all mounted in the same plane and that each carries the same polarizing charge per unit area. It will be clear that no matter what individual AC voltages and currents may be applied to the individual elements, provided that the total vector sum of the currents in the elements is frequency-independent the far field axis response will be frequency independent.

The foregoing conclusion applies equally to electrostatic loudspeaker elements of other constructional forms and to either push-pull or single-sided arrangements provided that in all cases the element concerned is acoustically transparent over the frequency range of interest, e.g. the full audio frequency range.

Thus in accordance with the present invention, an electrostatic loudspeaker that is at least substantially acoustically transparent over the frequency range of interest has separately accessible coplanar sections adapted to carry a common polarizing charge per unit area, these sections being so connected as the parallel components of an electrical network that, irrespective of the series branches of that network, the vector sum of the component currents in the individual sections will be equal to or proportional to the total current at the input terminals of the network.

Such a loudspeaker may have a single membrane or diaphragm with sub-divided signal electrode(s) or it may comprise a number of coplanar electrostatic loudspeaker units.

The series branches of the said network are preferably arranged to delay and/or attenuate the signal voltages applied to the individual parallel components so that the total radiating area and/or its effective phase contour varies with frequency, a well known technique to produce a desired directivity pattern.

A resistor may be connected to sample the current at the input terminals of the said network, the voltage developed being applied as feedback in accordance with known techniques to control the current, and hence the far field axis response, independent of changes of network impedance with frequency.

A typical embodiment of the invention is illustrated in the accompanying drawing in which:

FIG. 1 diagrammatically shows in front elevation an electrostatic loudspeaker of the constructional form discussed but modified in accordance with the invention; and

FIG. 2 illustrates the circuitry associated with the loudspeaker of FIG. 1.

The loudspeaker illustrated in the drawings comprises a circular diaphragm 1 supported between a pair of electrodes 2, 3 that are each divided into concentric sections with the various sections connected by impedances 4. In this respect, the arrangement is equivalent to that of a group of serially connected loudspeaker units with impedances therebetween, the purpose of which is both to provide an improved impedance match to the amplifier and to provide an improved directivity characteristic. However, in known arrangements of serially connected units, the objective of a level axis response is sought by a. designing each unit to cover a small part of the audio frequency range with a more or less level response and the required sensitivity in that range and then designing the network to distribute the frequency spectrum accordingly; and/or b. connecting the units in the form of a delay line terminated with a resistor equal to the characteristic impedance of the line, so that the axis response will be uniform over that part of the frequency range where the ratio of the acoustic radiation resistance to the square of the total load impedance can be held constant. There is, therefore, considerable deviation from level response in the region where the diameter of the loudspeaker is about one-third of a wavelength.

Both these expedients depend for success upon accurate control of the series impedances.

In the arrangement of the invention, as typified by that illustrated, however, the vector sum of the currents through the sections is monitored and with charge uniformity and acoustic transparency of each section a level axis response is obtained completely independent of wavelength to loudspeaker dimensions and completely independent of accurate control of the series impedances. It is, of course, desirable to maintain reasonable control of the series impedances to avoid deviation in the polar response.

Charge uniformity is conveniently achieved by the expedient of employing a diaphragm of high resistivity as more fully explained in U.S. Pat. Nos. 3008013 and 3008014: in FIG. 2, the diaphragm l is shown connected to a polarizing voltage source 5 through a resistor 6 but in practice the resistance represented by resistor 6 is distributed in the diaphragm 1. Acoustic transparency is obtained, for the frequency range of interest, by appropriate selection of diaphragm tension and mass as above mentioned.

A convenient method of controlling the vector sum of the currents in the individual sections is by means of a resistor 7 to sample the current flowing to the sections of one electrode, e.g. electrode 2., and to develop a voltage that may be applied via line 8 as feedback to the amplifier that feeds the loudspeaker through the usual transformer shown at 9. Care must be taken to keep stray capacitance to a minimum to ensure that the current through the resistor 7 is indeed proportional to the true vector sum of unit currents. In a practical loudspeaker, virtually complete and automatic control of axis response may be obtained from about 1001-12 to 10,000Hz. The high frequency limit is due to stray capacity and lack of acoustic transparency. Clearly if these limits are measured or calculated, corrections can be applied either by replacing the feedback resistor by a more complex network or by applying an electrical correction circuit external to the loop. The low frequency limit is due to lack of acoustic transparency. This is associated with the fundamental resonance of the diaphragm l at say 40 c/s and this must be damped electrically and/or mechanically as would be done with a conventional electrostatic loudspeaker.

What is claimed is:

1. An electrostatic loudspeaker system that is at least substantially acoustically transparent over a predetermined frequency range and comprising, a pair of loudspeaker terminals, a plurality of separately accessible coplanar voltage driven sections operatively connected to have a common polarising charge per unit area applied thereto, an electrical network connecting said sections to said terminals such as to improve the non-directionability of the loudspeaker at high audio frequencies, a pair of input terminals, constant-current driving means connected between said pair of input terminals and said loudspeaker terminals, the voltage across said loudspeaker terminals driving the loudspeaker in response to an audio signal applied to said input terminals, the far field axis-pressure response of said loudspeaker system being directly related to the driver current of said driving means, and means for controlling said driving means in a manner such that the vector sum of all the currents in said coplanar sections is made to follow the instantaneous amplitude of said audio signal applied to said input terminals.

2. An electrostatic loudspeaker system in accordance with claim 1, including a single diaphragm operatively associated with said coplanar sections, and each of said sections including a pair of electrodes disposed on opposite sides of said diaphragm.

3. An electrostatic loudspeaker system in accordance with claim 1, wherein said means connecting said sections to said input terminals comprise a network in which said sections are parallel-connected components, said network having series branches arranged in a manner to produce a predetermined directivity pattern.

4. An electrostatic loudspeaker system in accordance with claim 1, wherein said driving means includes a sampling resistor connected at said input terminals, the voltage developed across said resistor being applied as feedback to maintain the current independent of changes of the impedance with frequency.

5. An electrostatic loudspeaker system in accordance with claim 1, including a flexible diaphragm of high resistivity, each ofsaid sections including a pair of parallel signal electrodes disposed one on each side of said diaphragm, and each of said seconds being of equal area and being electrically isolated from one another.

6. An electrostatic loudspeaker in accordance with claim 1 wherein said controlling means includes means for monitoring the vector sum of all the currents in said coplanar sections, and a feedback connection to said driving means for maintaining said vector sum directly proportional to the voltage of said audio signal applied to said input terminals.

7. An electrostatic loudspeaker according to claim 6, wherein said sections are connected as parallel impedances in said network and said monitoring means comprise a single resistor connected in series in the network to carry the total current flowing in said sections, thereby to develop a feedback voltage thereacross proportional to said total current. 

1. An electrostatic loudspeaker system that is at least substantially acoustically transparent over a predetermined frequency range and comprising, a pair of loudspeaker terminals, a plurality of separately accessible coplanar voltage driven sections operatively connected to have a common polarising charge per unit area applied thereto, an electrical network connecting said sections to said terminals such as to improve the nondirectionability of the loudspeaker at high audio frequencies, a pair of input terminals, constant-current driving means connected between said pair of input terminals and said loudspeaker terminals, the voltage across said loudspeaker terminals driving the loudspeaker in response to an audio signal applied to said input terminals, the far field axis-pressure response of said loudspeaker system being directly related to the driver currEnt of said driving means, and means for controlling said driving means in a manner such that the vector sum of all the currents in said coplanar sections is made to follow the instantaneous amplitude of said audio signal applied to said input terminals.
 2. An electrostatic loudspeaker system in accordance with claim 1, including a single diaphragm operatively associated with said coplanar sections, and each of said sections including a pair of electrodes disposed on opposite sides of said diaphragm.
 3. An electrostatic loudspeaker system in accordance with claim 1, wherein said means connecting said sections to said input terminals comprise a network in which said sections are parallel-connected components, said network having series branches arranged in a manner to produce a predetermined directivity pattern.
 4. An electrostatic loudspeaker system in accordance with claim 1, wherein said driving means includes a sampling resistor connected at said input terminals, the voltage developed across said resistor being applied as feedback to maintain the current independent of changes of the impedance with frequency.
 5. An electrostatic loudspeaker system in accordance with claim 1, including a flexible diaphragm of high resistivity, each of said sections including a pair of parallel signal electrodes disposed one on each side of said diaphragm, and each of said seconds being of equal area and being electrically isolated from one another.
 6. An electrostatic loudspeaker in accordance with claim 1 wherein said controlling means includes means for monitoring the vector sum of all the currents in said coplanar sections, and a feedback connection to said driving means for maintaining said vector sum directly proportional to the voltage of said audio signal applied to said input terminals.
 7. An electrostatic loudspeaker according to claim 6, wherein said sections are connected as parallel impedances in said network and said monitoring means comprise a single resistor connected in series in the network to carry the total current flowing in said sections, thereby to develop a feedback voltage thereacross proportional to said total current. 